There are various types of actuators that incorporate piezoelectric elements. These actuators utilize different modes of operation, referred to in the industry as the d33 or d31 operating modes, depending upon the direction of expansion or contraction of the piezoelectric material relative to the direction of the electric field that is applied to same. The relative displacement of the piezoelectric material in the d33 mode of operation is approximately two times greater than the displacement of same in the d31 operating mode. Displacement of piezoelectric material in the d33 operating mode is in the form of expansion in the same direction as the applied electrical field and poling direction, whereas displacement of such material in the d31 operating mode is in the form of contraction in a direction perpendicular to the applied electrical field and poling direction.
Stack type piezoelectric actuators (d33 actuators) are solid-state linear devices. As such, these actuators utilize the expansion of piezoelectric material to produce a positive displacement. In general, the active part of these actuators consists of a stack of ceramic layers separated by thin metallic layers which act as electrodes. A typical stack type actuator may produce a deflection of about 0.002 inches, a force of about 200 lbs. and work of about 0.4 in-lbs. Thus, d33 mode actuators provide a large amount of work, however, they require a relatively complex assembly, a large package size, and a sophisticated, high cost power supply. In addition, these actuators possess excessive capacitances and hysteresis. Furthermore, these actuators are relatively expensive to produce and are heavy.
Contraction type actuators (d31 actuators) utilize the contraction of piezoelectric material to produce a negative displacement. The piezoelectric material when bonded to a metallic strip exhibits a bending motion as it contracts. A bending type d31 mode actuator may consist of a single layer of piezoelectric material bonded to a metallic strip or several layers of bonded pairs. The displacement of such material provided by d31 actuators, which is perpendicular to the direction of the applied electrical field, is a function of the length of the actuator. The number of ceramic layers utilized determines the resulting stiffness and output force of the actuator. The layers or “bimorph” strips can produce a relatively large deflection in a relatively small, low cost package, however, these actuators are severely limited in their ability to produce a force. For example, a typical d31 mode actuator may produce a deflection of about 0.1 inches, a force of about 0.08 lbs. and work of about 0.008 in-lbs. Because these actuators are severely limited in their ability to produce force, they cannot be used in those applications that require a relatively large force, such as 1-100 pounds.
In view of the foregoing, it has become desirable to develop a piezoelectric actuator that can be operated in the d31 mode to obtain the deflection advantages of this operating mode and which controls the potential energy of a spring to increase the force and work produced by the actuator.